The actress's character Michelle Connor did not feature in last week's special storyline, which saw disaster hit the cobbles when The Joinery exploded and a tram careered off the viaduct.
Marsh - who is engaged to Hollyoaks actor Jamie Lomas - announced her pregnancy back in October after having her 12-week scan.
Writing in her latest New magazine column, Marsh explained: "The Corrie 50th anniversary episodes went down a storm and everyone was delighted with the reaction. You probably noticed that I didn't appear, and that's because there were stunts involved."
The star - who already has two children from a previous relationship - continued: "Because of my pregnancy, the producers thought it would be better if I didn't take part. They explained my absence by saying Michelle had gone to see Ryan in Glasgow. Of course, I was a little disappointed not to be part of it, but there's no contest when it comes to the wellbeing of my baby.
"It was great to watch the episodes as a regular viewer and I also really enjoyed the screening of the first ever episode - the humour was evident from the beginning!"
Marsh and Lomas suffered the loss of their first child together in February 2009. Baby boy Archie Jay passed away in hospital moments after he was born 18 weeks premature.